The kraft process is currently the dominant chemical pulping process. During pulping large quantities of recoverable energy in the form of black liquor is generated. Worldwide some 2.8 billion GJ (780 TWh) of black liquor was produced in 1990 at kraft pulp mills.
The kraft recovery system has two principal functions:
i) Recovery and recure of the inorganic pulping chemicals. PA1 ii) Recovery of the energy value of the organic material as process steam and electrical power.
The chemical recovery process contributes significantly to the capital intensity of the kraft process. About 35% of the capital cost of a modern pulp mill is attributable to the recovery process.
The predominant method today for recovery of chemicals and energy from black liquor is the Tomlinson recovery boiler, a technology which was introduced well over fifty years ago. Although an established technology, there are some wellknown disadvantages with conventional recovery technology.
Most often the recovery boiler with its inherent in flexibility constitutes the main production bottleneck in the pulp mill. Economics of scale dictate large capacity units.
Other disadvantages include the low thermal efficiency and risk of smelt water explosions which in turn con stitute a safety problem.
These and other areas of concern have been the driving force for development of new methods and principles for recovering chemicals and energy from black liquor. One of the more promising routes is gasification of the liquor in entrained or fluidized beds. In some cases these alternative processes can be installed as incremental capacity boosters, providing an opportunity to eliminate the recovery boiler bottleneck.
One of the major driving forces for development of new recovery technology has been to improve thermal efficiency accompanied with higher power to steam output ratios. The present invention relates to a major improvement in this area, using technology based on gasification and energy recovery in a recuperated gas turbine cycle.
Gasification of black liquor can be performed at various temperatures and pressures, resulting in different forms of the recovered inorganic constituents and different calorific values of the combustible process gas.
The inorganics, mainly sodium compounds, are solubilized to form an aqueous alkaline liquid called green liquor, which liquor is used for cooking liquor preparation.
Kraft pulp mills are significant producers of biomass energy and today most mills are designed to use the biomass fuel available at the kraft mill to meet on site steam and electricity needs via back pressure steam turbine cogeneration system. Electricity demand is often higher than internally generated, in particular for integrated mills and often electricity is imported from the grit.
Process steam requirements for a modern kraft pulp mill is in the order of 10 GJ per ton of air dried pulp. The internal electricity demand is around 600 kWh/ton of air dried pulp.
The biomass gasification gas turbine cogeneration system of the present invention will meet mill steam demand and has the potential to produce excess electricity for export.
The present invention can be practised using various types of gas generators and gasification principles exemplified in prior art documents.
In U.S. Pat. No. 4,917,763 and U.S. Pat. No. 4,692,209, gasification of spent cellulose liquor, such as black liquor, is described. The gasification temperature is in the range of 1000.degree.-1300.degree. C., resulting in the evolvement of molten inorganics and a combustible gas. The molten alkaline chemicals are withdrawn from the gas stream in a cooling and quenching stage where an aqueous solution is sprayed into the gas steam. The product alkaline solution is cooled to below 200.degree. C.
The combustible gas is used for generating steam or as a synthesis gas.
Another gasification method is described in U.S. Pat. No. 4,808,264 where recovery and energy from black liquor is carried out in three distinct and separate steps, whereas in the first step concentrated black liquor is gasified in a pressurized gasification reactor by flash pyrolysis at 700.degree. to 1300.degree. C., in which the inorganic chemicals of the black liquor are contained in the form of molten suspended droplets.
Energy is recovered from the resulting process gas for generation of steam and/or electric power in a gas turbine/steam turbine cycle. The steam turbine is of back pressure type preferably selected to fit the needs of process steam for the mill.
In WO 91/15665 is described a method and apparatus for generation of electricity and steam from a pressurized black liquor gasification process. Energy is recovered in a gas turbine/back pressure steam turbine system. Excess steam generated in the mill is recirculated into the gas turbine or the combustor thereof for increasing the generation of electricity. This procedure is known to the industry as a steam injected gas turbine hereinafter referred to as STIG.
Common for U.S. Pat. No. 4,808,264 and WO 91/15665 is that they both are based on energy recovery using a combined cycle including a back pressure steam turbine.
These systems have a rather high thermal efficiency but suffer from the high capital cost of the steam turbine and waste heat steam generator. The electricity output from a condensing steam turbine in a combined cycle is often less than a third of the total power output and considerably less for back pressure turbines.
A bottoming steam cycle as in these inventions has an inherent high thermodynamic irreversiblity since the evaporation of water occurs at constant temperature, whereas the heat release occurs at varying temperatures, leading to lower thermal efficiencies.
The objective of the present invention is to provide a process for more efficient and less capital intensive production of electric power and process steam from gasification of black liquor using a recuperated gas turbine cycle following a gas quench cooler and heat exchange system where the hot process gas from the gasifier reaction zone is cooled to a temperature below 150.degree. C., simultaneously recovering sensible and latent heat transferred for generation of steam for mill internal use.
A substantial quantity of sensible heat can also be extracted from hot liquids such as quench liquids, condensates and coolants, discharged from or within the quench zone and/or heat exchange zones.
Recovery of latent and sensible heat can be performed in various types of equipment including heat exchange steam generators, boiler feed water heaters and heat pumps.
In a specific embodiment, latent and/or sensible heat in the gas and/or liquid streams is recovered using a reversed absorption heat pump, where a heat absorbing medium such as for example sodium hydroxide solution is used for heat transfer.
The cooled combustible process gas is transferred to a gas turbine system in which some or all of the excess air, which is used as thermal diluent and working fluid, is replaced with water vapor.
Gas turbines are very sensitive to contaminants in the incoming gas stream, in particular sulfur oxides and alkali salts. To prevent harmful effects on turbo machinery, the gases have to be substantially free from these and other contaminants, in particular if the gas is used as fuel in an internally fired gas turbine cycle. It is therefore important to have efficient gas cleaning in the present invention in particular with respect to sodium, as sodium is a dominant inorganic compound in cellulose waste liquors.
It is appreciated that substantially all vaporized sodium compounds and particulates are removed in the quench gas cooler and scrubbing system of the present invention. Saturation vapour pressure of the harmful components in question is very low at temperatures below 200.degree. C.
If necessary the process gas can be filtered or sodium compounds can be sorbed on an appropriate involatile inorganic sorbent, such as an alumino-silicate before the gas enters the gas turbine combustor. Zeolites may be used as filters or as sorbant surface for alkali removal.
One way to get around this problem totally is to use the process gas as fuel in an externally fired gas turbine cycle, which is another optional embodiment of the present invention, described subsequently herein.
Although gas turbine cycles have inherent thermodynamic advantages, simple cycle gas turbine systems suffer from some well known disadvantages as well, such as the large parasitic load of cooling air on the system to decrease the turbine inlet temperature.
Furthermore, the exhaust from the gas turbine contains a large quantity of sensible heat and, if discharged to atmosphere, large quantities of potentially useful energy are wasted. However, this exhaust heat can be exploited in various ways, for example to produce steam in a heat recovery steam generator (HRSG), which can be used for process needs directly or in a cogeneration figuration, or to produce more power in a condensing steam turbine. In light of the strong scale economics of steam turbine cycles and other factors described herein, combined gas turbine and steam turbine cycles based on heavy duty industrial turbines are not the best candidates for applications in the relatively modest scales in conjunction with black liquor gasification.
Another method to exploit the heat content of turbine exhaust is to raise superheated steam which is recirculated and injected in the combustor of the combustion turbine, see e.g. U.S. Pat. No. 3,978,661. Steam injection in biomass gasifier gas turbine cogeneration systems for forest product industry applications is for example described in PU/CEES Working Paper No 113 by Dr Eric Larson, Princeton, February 1990.
Yet another method to exploit the turbine exhaust is to preheat the air leaving the compressor against engine exhaust in a recuperative heat exchanger and simultaneously use interstage cooling during air compression. Injection of water in a recuperative cycle can further improve efficiency.
The principle of water injected recuperative gas turbine cycles is previously described, for instance in U.S. Pat. No. 2,869,324 and U.S. Pat. No. 4,537,023, and in literature; Gasparovic N., "Gas turbines with heat exchanger and water injection in the compressed air", Proc. Instn. Mech. Engrs., vol. 185, 1971.
A major drawback of direct fired gas turbine cycles as exemplified in prior art documents above is the high sensitivity to fuel gas quality.
Indirectly fired or externally fired gas turbine cycles are considerably less sensitive and can accept fuels of approximately the same quality as steam generators.
Indirect cycles, currently under development for coal gasification applications, can accommodate a wide variety of conventional equipment. Advanced combustors and high temperature heat exchangers are commercially available or under development.
Stack gas recirculation to use all the cycle air for combustion can be attractive in indirect cycles, minimizing NO.sub.x emissions and lowering capital cost.
As will be subsequently explained herein, use of an indirectly fired gas turbine cycle in combination with compressed air humidification by water injection is an attractive alternative embodiment of the present invention.
The practise of the present invention will be described by reference to the appended description, examples and figures as applied to the recovery from black liquor. It should, however, be recognized that the invention is applicable to the recovery of other cellulose waste liquors, such as for example spent sulfite or soda pulping liquors.